The addition of a complexant such as thiourea or derivatives thereof is required whenever tin or a tin alloy is deposited by an immersion plating process on copper substrates. The role of thiourea is to support the dissolution of copper by forming Cu(I) thiourea complexes during the immersion reaction with Sn(II) ions. As copper is more noble than tin such a support reaction is required to reduce Sn(II) ions by oxidation of copper.
On the other hand the concentration of Cu(I) ions and Cu(I) thiourea complex is increased in the plating bath during use of a tin or tin alloy immersion plating process. When saturation of the Cu(I) thiourea complex in the immersion tin plating bath is exceeded said Cu(I) thiourea complex starts to form undesired precipitations in the plating equipment, e.g., in spray nozzles and other mechanical components.
Furthermore, copper ions in an immersion tin plating bath can reverse the desired reaction of tin deposition, i.e., by dissolving the tin layer and deposition of metallic copper.
Acidic immersion tin plating baths comprising thiourea or derivatives thereof are known since a long time (The Electrodeposition of Tin and its Alloys, M. Jordan, Eugen G. Leuze Publishers, 1995, pages 89 to 90 and references cited therein).
An acidic immersion tin plating bath comprising thiourea and optionally a surfactant which can be a polyalkylene glycol compound is disclosed in JP 9-302476 A. A Cu(I) thiourea complex precipitated from such plating bath compositions lead to voluminous precipitates which tend to block spray nozzles, filters and other mechanical components of the plating equipment during use of the plating bath and during removal of the precipitated complex. Furthermore, the formation of a Cu(I) thiourea complex compounds from dissolved Cu(I) ions in the plating bath is not completely. Dissolved Cu(I) ions remain in the plating bath at all times during use. Said free Cu(I) ions in the plating bath are prone to reverse tin deposition. This effect is problematic in case the deposited tin layer should serves to provide a solderable or bondable surface for electronic devices.
A method to remove precipitates of a Cu(I) thiorurea complex from acidic immersion tin plating baths is disclosed in U.S. Pat. No. 5,211,831 wherein a portion of a immersion tin plating bath in use is transferred from the plating tank to a separate crystallization unit. The still dissolved Cu(I) thiourea complex is selectively precipitated in the separate crystallization unit by cooling down said portion and the remaining tin plating bath portion is transferred back to the plating tank. Such methods comprise a filtration step wherein the precipitated Cu(I) thiourea complex is removed from the immersion tin plating bath by filtering off the precipitate.